Piezoelectric power generation device

ABSTRACT

A small-sized piezoelectric power generation device is provided which can generate electric power even with vibrations at low frequencies. The piezoelectric power generation device includes a piezoelectric element formed in a spiral shape and having a center-side end fixed to a first fixing member and an outer-side end fixed to a second fixing member. The piezoelectric element includes a first piezoelectric body formed in a spiral shape and polarized in a radial direction of the piezoelectric element from one side toward the other side. The first and second fixing members are arranged such that the second fixing member is displaceable relative to the first fixing member in a tangential direction x, whereas the second fixing member is restricted from displacing relative to the first fixing member in a perpendicular direction y.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric power generationdevice, and more particularly to a piezoelectric power generation devicewhich can generate electric power with vibrations (oscillations) at lowfrequencies of not higher than about 20 Hz, for example.

2. Description of the Related Art

Hitherto, there is known a piezoelectric power generation device whichcauses vibrations upon application of accelerations or strains andgenerates electric power based on the piezoelectric effect. When thepiezoelectric power generation device is employed as a power supply,another additional power supply, such as a battery, is no longerrequired. Accordingly, the above-mentioned piezoelectric powergeneration device is suitably used as, e.g., a power supply for varioussensors which are employed in situations where vibrations or strains areapplied.

One example of the above-mentioned piezoelectric power generation deviceis disclosed in Japanese Patent No. 3170965. FIG. 19 is a perspectiveview of a piezoelectric power generation device described in JapanesePatent No. 3170965. A piezoelectric power generation device 100illustrated in FIG. 19 is a unimorph-type piezoelectric power generationdevice in the form of cantilevered beam. More specifically, asillustrated in FIG. 19, the piezoelectric power generation device 100includes a piezoelectric element 102. A metal plate 103 is joined to thepiezoelectric element 102. Respective one ends of the piezoelectricelement 102 and the metal plate 103 are fixed to a case 101. A weight104 is attached to the other ends of the piezoelectric element 102 andthe metal plate 103. Upon acceleration being applied to thepiezoelectric power generation device 100, the weight 104 is caused tovibrate. With the vibration of the weight 104, tensile stress andcompressive stress are alternately imposed to the piezoelectric element102. As a result, the piezoelectric element 102 generates electricpower.

In the piezoelectric power generation device 100, the resonancefrequency of the piezoelectric element 102 is correlated to both thelength of the piezoelectric element 102 and the weight of the weight104. More specifically, as the length from the fixed end to the free endof the piezoelectric element 102 increases, the resonance frequency ofthe piezoelectric element 102 lowers. Also, as the weight of the weight104 increases, the resonance frequency of the piezoelectric element 102lowers.

In order to generate electric power with vibrations at low frequencies,therefore, it is required to lower the resonance frequency of thepiezoelectric element 102 by increasing the length of the piezoelectricelement 102 and the weight of the weight 104. Thus, trying to generateelectric power with vibrations at low frequencies causes a problem thatthe size of the piezoelectric power generation device 100 necessarilyincreases. In particular, the frequency of a motion of a human body,e.g., walking, is usually as very low as about 20 Hz or less.Accordingly, when the low-frequency vibration provided by the motion ofthe human body is utilized to generate electric power, a problem arisesin that the piezoelectric power generation device 100 is required tohave a very large size.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide a piezoelectric power generation device whichhas a small size and which can generate electric power even withvibrations at frequencies as low as about 20 Hz or less, for example.

A piezoelectric power generation device according to a first preferredembodiment of the present invention includes a piezoelectric element, afirst fixing member, and a second fixing member. The piezoelectricelement is formed substantially in a spiral shape. A center-side end ofthe piezoelectric element is fixed to the first fixing member. Anouter-side end of the piezoelectric element is fixed to the secondfixing member. The piezoelectric element includes a first piezoelectricbody, a first electrode, a second electrode, and an elastic member. Thefirst piezoelectric body is formed substantially in a spiral shape. Thefirst piezoelectric body is polarized in a radial direction from oneside toward the other side. The first electrode is disposed on an innersurface of the first piezoelectric body as viewed in the radialdirection. The second electrode is disposed on an outer surface of thefirst piezoelectric body as viewed in the radial direction. The elasticmember is disposed on an outer surface of the second electrode as viewedin the radial direction or an inner surface of the first electrode asviewed in the radial direction. The first and second fixing members areconstituted such that the second fixing member is displaceable relativeto the first fixing member in a direction tangential to the outer-sideend of the piezoelectric element, whereas the second fixing member isrestricted from displacing relative to the first fixing member in adirection that is perpendicular to the direction tangential to theouter-side end of the piezoelectric element.

In one particular aspect of the piezoelectric power generation deviceaccording to the first preferred embodiment of the present invention,the elastic member is a second piezoelectric body which is polarized inthe radial direction from the other side toward the one side, and thepiezoelectric element further includes a third electrode disposed on asurface of the second piezoelectric body. With such an arrangement, ahigher level of power generation efficiency can be obtained.

In another particular aspect of the piezoelectric power generationdevice according to the first preferred embodiment of the presentinvention, the first fixing member includes a plate-like member havingone surface on which the piezoelectric element is positioned, and aprojected portion projecting from the one surface of the plate-likemember at a position outwards of the outer-side end of the piezoelectricelement as viewed in the radial direction, the second fixing member ispositioned on the side closer to the piezoelectric element with respectto the projected portion and has a contact surface extending in thedirection tangential to the outer-side end of the piezoelectric element,and the contact surface is held in contact with the projected portion.With such an arrangement, the second fixing member can be effectivelyrestricted from displacing relative to the first fixing member in theradial direction of the piezoelectric element, and the displacement ofthe second fixing member relative to the first fixing member in thedirection tangential to the outer-side end of the piezoelectric elementcan be increased. Hence, a higher level of power generation efficiencycan be realized.

In still another particular aspect of the piezoelectric power generationdevice according to the first preferred embodiment of the presentinvention, the second fixing member includes a fixing member body havingthe contact surface, a first stopper portion connected to the fixingmember body and positioned on one side with respect to the projectedportion as viewed in the direction tangential to the outer-side end ofthe piezoelectric element, and a second stopper portion connected to thefixing member body and positioned on the other side with respect to theprojected portion as viewed in the direction tangential to theouter-side end of the piezoelectric element. With such an arrangement,the second fixing member can be restricted from excessively displacingrelative to the first fixing member in the tangential direction. Hence,the piezoelectric element can be effectively protected against damagethat may be otherwise caused by large stress imposed to thepiezoelectric element.

A piezoelectric power generation device according to a second preferredembodiment of the present invention includes first and secondpiezoelectric elements, a plate-like first fixing member, and a secondfixing member. The first and second piezoelectric elements are eachformed substantially in a spiral shape. The first piezoelectric elementand the second piezoelectric element are arranged on one surface of thefirst fixing member. A center-side end of the first piezoelectricelement and a center-side end of the second piezoelectric element areboth fixed to the first fixing member. The second fixing member ispositioned on the one surface of the first fixing member between thefirst piezoelectric element and the second piezoelectric element. Aportion of the first piezoelectric element positioned closest to thesecond piezoelectric element and a portion of the second piezoelectricelement positioned closest to the first piezoelectric element are bothfixed to the second fixing member on a straight line passing a center ofthe first piezoelectric element and a center of the second piezoelectricelement. The second fixing member is displaceable relative to the firstfixing member. Each of the first and second piezoelectric elementsincludes a first piezoelectric body, a first electrode, a secondelectrode, and an elastic member. The first piezoelectric body is formedsubstantially in a spiral shape. The first piezoelectric body ispolarized in a radial direction from one side toward the other side. Thefirst electrode is disposed on an inner surface of the firstpiezoelectric body as viewed in the radial direction. The secondelectrode is disposed on an outer surface of the first piezoelectricbody as viewed in the radial direction. An elastic member is disposed onan outer surface of the second electrode as viewed in the radialdirection or an inner surface of the first electrode as viewed in theradial direction.

In one particular aspect of the piezoelectric power generation deviceaccording to the second preferred embodiment of the present invention,the elastic member is a second piezoelectric body which is polarized inthe radial direction from the other side toward the one side, and eachof the first and second piezoelectric elements further includes a thirdelectrode disposed on a surface of the second piezoelectric body. Withsuch an arrangement, a higher level of power generation efficiency canbe obtained.

In another particular aspect of the piezoelectric power generationdevice according to the second preferred embodiment of the presentinvention, an outer-side end of the first piezoelectric element and anouter-side end of the second piezoelectric element are positioned on aportion of the straight line passing the center of the firstpiezoelectric element and the center of the second piezoelectricelement, which portion is located between the center of the firstpiezoelectric element and the center of the second piezoelectricelement, and the outer-side end of the first piezoelectric element andthe outer-side end of the second piezoelectric element are fixed to thesecond fixing member. With such an arrangement, stresses are imposed tothe entirety of the first and second piezoelectric bodies upon thesecond fixing member displacing relative to the first fixing member,whereby the first and second piezoelectric bodies are entirely deformed.Hence, the power generation efficiency can be further increased.

A piezoelectric power generation device according to a third preferredembodiment of the present invention includes a piezoelectric element, afirst fixing member, and a second fixing member. The piezoelectricelement is formed substantially in a spiral shape. A center-side end ofthe piezoelectric element is fixed to the first fixing member. Anouter-side end of the piezoelectric element is fixed to the secondfixing member. The piezoelectric element includes a first piezoelectricbody, a first electrode, a second electrode, and an elastic member. Thefirst piezoelectric body is formed is formed substantially in a spiralshape. The first electrode is disposed on an inner surface of the firstpiezoelectric body as viewed in the radial direction. The secondelectrode is disposed on an outer surface of the first piezoelectricbody as viewed in the radial direction. An elastic member is disposed onan outer surface of the second electrode as viewed in the radialdirection or an inner surface of the first electrode as viewed in theradial direction. A portion of the first piezoelectric body, which islocated on one side of a straight line passing a center of thepiezoelectric element and the outer-side end of the piezoelectricelement, is polarized in the radial direction of the piezoelectricelement from one side toward the other side, and a portion of the firstpiezoelectric body, which is located on the other side of the straightline passing the center of the piezoelectric element and the outer-sideend of the piezoelectric element, is polarized in the radial directionof the piezoelectric element from the other side toward the one side.The first and second fixing members are constituted such that the secondfixing member is displaceable relative to the first fixing member in adirection in which the straight line passing the center of thepiezoelectric element and the outer-side end of the piezoelectricelement extends, whereas the second fixing member is restricted fromdisplacing relative to the first fixing member in a direction tangentialto the outer-side end of the piezoelectric element.

In one particular aspect of the piezoelectric power generation deviceaccording to the third preferred embodiment of the present invention,the elastic member is a second piezoelectric body. A portion of thesecond piezoelectric body, which is located on one side of the straightline passing the center of the piezoelectric element and the outer-sideend of the piezoelectric element, is polarized in the radial directionof the piezoelectric element from the other side toward the one side. Aportion of the second piezoelectric body, which is located on the otherside of the straight line passing the center of the piezoelectricelement and the outer-side end of the piezoelectric element, ispolarized in the radial direction of the piezoelectric element from theone side toward the other side. The piezoelectric element furtherincludes a third electrode disposed on a surface of the secondpiezoelectric body.

In still another particular aspect of the piezoelectric power generationdevice according to the third preferred embodiment of the presentinvention, the first fixing member includes a plate-like member havingone surface on which the piezoelectric element is positioned, and aprojected portion projecting from the one surface of the plate-likemember at a position outwards of the outer-side end of the piezoelectricelement as viewed in the radial direction, the second fixing memberincludes a first guide portion extending in the direction in which thestraight line passing the center of the piezoelectric element and theouter-side end of the piezoelectric element extends, and positioned onone side with respect to the projected portion as viewed in thedirection tangential to the outer-side end of the piezoelectric element,and a second guide portion extending in the direction in which thestraight line passing the center of the piezoelectric element and theouter-side end of the piezoelectric element extends, and positioned onthe other side with respect to the projected portion as viewed in thedirection tangential to the outer-side end of the piezoelectric element,and the first and second guide portions are each held in contact withthe projected portion. With such an arrangement, the second fixingmember can be effectively restricted from displacing relative to thefirst fixing member in the direction tangential to the outer-side end ofthe piezoelectric element, and the displacement of the second fixingmember relative to the first fixing member in the radial direction ofthe piezoelectric element can be increased. Hence, a higher level ofpower generation efficiency can be realized.

A piezoelectric power generation device according to a fourth preferredembodiment of the present invention includes first and secondpiezoelectric elements, a plate-like first fixing member, and a secondfixing member. The first and second piezoelectric elements are eachformed substantially in a spiral shape. The first piezoelectric elementand the second piezoelectric element are arranged on one surface of thefirst fixing member so as to line up in one direction. A center-side endof the first piezoelectric element and a center-side end of the secondpiezoelectric element are both fixed to the first fixing member. Asecond fixing member is arranged on the one surface of the first fixingmember to be displaceable relative to the first fixing member. Thesecond fixing member includes a connecting portion and a weight portion.The connecting portion interconnects a portion of the firstpiezoelectric element, which is positioned closest to one side in asecond direction perpendicular to the first direction, and a portion ofthe second piezoelectric element, which is positioned closest to the oneside in the second direction. The weight portion extends from theconnecting portion in the second direction and is positioned between acenter of the first piezoelectric element and a center of the secondpiezoelectric element as viewed in the first direction. Each of thefirst and second piezoelectric elements includes a first piezoelectricbody, a first electrode, a second electrode, and an elastic member. Thefirst piezoelectric body is formed substantially in a spiral shape. Thefirst electrode is disposed on an inner surface of the firstpiezoelectric body as viewed in the radial direction. The secondelectrode is disposed on an outer surface of the first piezoelectricbody as viewed in the radial direction. The elastic member is disposedon an outer surface of the second electrode as viewed in the radialdirection or an inner surface of the first electrode as viewed in theradial direction.

Further, a portion of the first piezoelectric body of the firstpiezoelectric element, which is located on one side of a straight linepassing a center of the first piezoelectric element and an outer-sideend of the first piezoelectric element, is polarized in the radialdirection of the first piezoelectric element from one side toward theother side. A portion of the first piezoelectric body of the firstpiezoelectric element, which is located on the other side of thestraight line passing the center of the first piezoelectric element andthe outer-side end of the first piezoelectric element, is polarized inthe radial direction of the first piezoelectric element from the otherside toward the one side.

Still further, a portion of the first piezoelectric body of the secondpiezoelectric element, which is located on one side of a straight linepassing a center of the second piezoelectric element and an outer-sideend of the second piezoelectric element, is polarized in the radialdirection of the second piezoelectric element from one side toward theother side. A portion of the first piezoelectric body of the secondpiezoelectric element, which is located on the other side of thestraight line passing the center of the second piezoelectric element andthe outer-side end of the second piezoelectric element, is polarized inthe radial direction of the second piezoelectric element from the otherside toward the one side.

In one particular aspect of the piezoelectric power generation deviceaccording to the fourth preferred embodiment of the present invention,the elastic member is a second piezoelectric body, a portion of thesecond piezoelectric body of the first piezoelectric element, which islocated on one side of the straight line passing the center of the firstpiezoelectric element and the outer-side end of the first piezoelectricelement, is polarized in the radial direction of the first piezoelectricelement from the other side toward the one side, and a portion of thesecond piezoelectric body of the first piezoelectric element, which islocated on the other side of the straight line passing the center of thefirst piezoelectric element and the outer-side end of the firstpiezoelectric element, is polarized in the radial direction of the firstpiezoelectric element from the one side toward the other side. A portionof the second piezoelectric body of the second piezoelectric element,which is located on one side of the straight line passing the center ofthe second piezoelectric element and the outer-side end of the secondpiezoelectric element, is polarized in the radial direction of thesecond piezoelectric element from the other side toward the one side. Aportion of the second piezoelectric body of the second piezoelectricelement, which is located on the other side of the straight line passingthe center of the second piezoelectric element and the outer-side end ofthe second piezoelectric element, is polarized in the radial directionof the second piezoelectric element from the one side toward the otherside. Each of the first and second piezoelectric elements furtherincludes a third electrode disposed on a surface of the secondpiezoelectric body. With such an arrangement, a higher level of powergeneration efficiency can be obtained.

In another particular aspect of the piezoelectric power generationdevice according to the fourth preferred embodiment of the presentinvention, the outer-side end of the first piezoelectric element ispositioned on a portion of a straight line passing the center of thefirst piezoelectric element and extending in the second direction, whichportion is located on one side with respect to the center of the firstpiezoelectric element as viewed in the second direction, the outer-sideend of the second piezoelectric element is positioned on a portion of astraight line passing the center of the second piezoelectric element andextending in the second direction, which portion is located on one sidewith respect to the center of the second piezoelectric element as viewedin the second direction, and the connecting portion interconnects theouter-side end of the first piezoelectric element and the outer-side endof the second piezoelectric element. With such an arrangement, stressesare imposed to the entirety of the first and second piezoelectric bodiesupon the second fixing member displacing relative to the first fixingmember, whereby the first and second piezoelectric bodies are entirelydeformed. Hence, the power generation efficiency can be furtherincreased.

With the piezoelectric power generation devices according to the firstto fourth preferred embodiments of the present invention, since thepiezoelectric body is formed substantially in the spiral shape, thedistance between both the fixed ends of the piezoelectric body can beincreased without enlarging the size of the piezoelectric powergeneration device. In other words, the piezoelectric element having alonger size can be compactly stored in a smaller space by forming thepiezoelectric element substantially in the spiral shape. As a result,the piezoelectric power generation device capable of being driven withvibrations at low frequencies can be realized while the device size isheld small.

Also, with the piezoelectric power generation devices according to thefirst and second preferred embodiments of the present invention, thefirst piezoelectric body is polarized in the radial direction of thepiezoelectric element from the one side toward the other side, and thesecond fixing member to which the outer-side end of the piezoelectricelement is fixed is displaced just in the direction tangential to theouter-side end of the piezoelectric element relative to the first fixingmember to which the center-side end of the piezoelectric element isfixed. Therefore, electric power can be generated at a high level ofpower generation efficiency.

Further, with the piezoelectric power generation devices according tothe third and fourth preferred embodiments of the present invention, theportion of the first piezoelectric body, which is located on the oneside of the straight line passing the center of the piezoelectricelement and the outer-side end of the piezoelectric element, ispolarized in the radial direction of the piezoelectric element from theone side toward the other side, and the portion of the firstpiezoelectric body, which is located on the other side of the straightline passing the center of the piezoelectric element and the outer-sideend of the piezoelectric element, is polarized in the radial directionof the piezoelectric element from the other side toward the one side. Inaddition, the second fixing member to which the outer-side end of thepiezoelectric element is fixed is displaced just in the radial directionof the piezoelectric element relative to the first fixing member towhich the center-side end of the piezoelectric element is fixed.Therefore, electric power can be generated at a high level of powergeneration efficiency.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a piezoelectric powergeneration device according to a first embodiment;

FIG. 2 is a schematic perspective view of a piezoelectric element in thefirst embodiment;

FIG. 3 is a schematic sectional view of a portion cut out along a cutline III-III in FIG. 2;

FIG. 4 is a schematic plan view to explain the direction of polarizationin the piezoelectric element in the first embodiment (first to thirdelectrodes are omitted in FIG. 4 for convenience in drawing the figure);

FIGS. 5A and 5B are each a schematic explanatory view of thepiezoelectric element when reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element in the firstembodiment; specifically FIG. 5A is a schematic explanatory viewillustrating the direction of polarization in a portion A which islocated on the right side of a straight line L1 in FIG. 4 and stressesimposed to the right-side portion A, and FIG. 5B is a schematicexplanatory view illustrating the direction of polarization in a portionB which is located on the left side of the straight line L1 in FIG. 4and stresses imposed to the left-side portion B;

FIGS. 6A and 6B are each a schematic explanatory view of thepiezoelectric element when reciprocating vibration in the tangentialdirection x is applied to the piezoelectric element in the firstembodiment; specifically FIG. 6A is a schematic explanatory viewillustrating the direction of polarization in the portion A which islocated on the right side of the straight line L1 in FIG. 4 and stressesimposed to the right-side portion A, and FIG. 6B is a schematicexplanatory view illustrating the direction of polarization in theportion B which is located on the left side of the straight line L1 inFIG. 4 and stresses imposed to the left-side portion B;

FIG. 7 is a schematic perspective view of a piezoelectric powergeneration device according to a second embodiment;

FIG. 8 is a schematic plan view of the piezoelectric power generationdevice according to the second embodiment;

FIG. 9 is a schematic perspective view of a piezoelectric powergeneration device according to a third embodiment;

FIG. 10 is a schematic perspective view of a piezoelectric powergeneration device according to a fourth embodiment;

FIG. 11 is a schematic perspective view of a piezoelectric element inthe fourth embodiment;

FIG. 12 is a schematic sectional view of a portion cut out along a cutline XII-XII in FIG. 11;

FIG. 13 is a schematic plan view to explain the direction ofpolarization in the piezoelectric element in the fourth embodiment(first to third electrodes are omitted in FIG. 13 for convenience indrawing the figure);

FIGS. 14A and 14B are each a schematic explanatory view of thepiezoelectric element when reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element in the fourthembodiment; specifically FIG. 14A is a schematic explanatory viewillustrating the direction of polarization in a portion A which islocated on the right side of a straight line L1 in FIG. 13 and stressesimposed to the right-side portion A, and FIG. 14B is a schematicexplanatory view illustrating the direction of polarization in a portionB which is located on the left side of the straight line L1 in FIG. 13and stresses imposed to the left-side portion B;

FIGS. 15A and 15B are each a schematic explanatory view of thepiezoelectric element when reciprocating vibration in the tangentialdirection x is applied to the piezoelectric element in the fourthembodiment; specifically FIG. 15A is a schematic explanatory viewillustrating the direction of polarization in the portion A which islocated on the right side of the straight line L1 in FIG. 13 andstresses imposed to the right-side portion A, and FIG. 15B is aschematic explanatory view illustrating the direction of polarization inthe portion B which is located on the left side of the straight line L1in FIG. 13 and stresses imposed to the left-side portion B;

FIG. 16 is a schematic perspective view of a piezoelectric powergeneration device according to a fifth embodiment;

FIG. 17 is a schematic plan view to explain the direction ofpolarization in the piezoelectric element in a sixth embodiment(electrodes are omitted in FIG. 17 for convenience in drawing thefigure);

FIG. 18 is a schematic plan view to explain the direction ofpolarization in the piezoelectric element in a seventh embodiment(electrodes are omitted in FIG. 18 for convenience in drawing thefigure); and

FIG. 19 is a schematic perspective view of a piezoelectric powergeneration device described in Japanese Patent No. 3170965.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be apparent upon reading the followingdescriptions of preferred embodiments of the present invention withreference to the drawings.

First Embodiment

FIG. 1 is a schematic perspective view of a piezoelectric powergeneration device 1 according to a first embodiment. As illustrated inFIG. 1, the piezoelectric power generation device 1 includes apiezoelectric element 10 and first and second fixing members 20 and 30.The piezoelectric power generation device 1 generates electric power byutilizing vibration of the piezoelectric element 10 in the d31 mode.

The piezoelectric element 10 is formed substantially in a spiral(coiled) shape. More specifically, the piezoelectric element 10helically extends from a center of the spiral in a tangential directionperpendicular to a radial direction while gradually increasing thediameter of the spiral. A width W (see FIG. 3) of the piezoelectricelement 10 is smaller than a height H (see FIG. 3) of the piezoelectricelement 10.

As illustrated in FIGS. 2 and 3, the piezoelectric element 10 includesfirst and second piezoelectric bodies 11 and 12, and first to thirdelectrodes 13, 14 and 15. The first piezoelectric body 11 is formedsubstantially in the spiral shape. As illustrated in FIG. 3, a firstelectrode 13 is formed on an inner surface 11 a of the firstpiezoelectric body 11 as viewed in the radial direction. On the otherhand, a second electrode 14 is formed on an outer surface 11 b of thefirst piezoelectric body 11 as viewed in the radial direction. A secondpiezoelectric body 12 is disposed on an outer surface 14 a of the secondelectrode 14 as viewed in the radial direction. A third electrode 15 isformed on an outer surface 12 a of the second piezoelectric body 12 asviewed in the radial direction. Stated another way, the first electrode13, the first piezoelectric body 11, the second electrode 14, the secondpiezoelectric body 12, and the third electrode 15 are successivelyformed in the named order in the radial direction of the piezoelectricelement 10 from the inner side toward the outer side thereof. Thus, thefirst piezoelectric body 11 is sandwiched between the first electrode 13and the second electrode 14. Electric power generated in the firstpiezoelectric body 11 is taken out through a lead wire 16 a (see FIG. 2)connected to the first electrode 13 and a lead wire 16 b (see FIG. 2)connected to the second electrode 14. On the other hand, the secondpiezoelectric body 12 is sandwiched between the second electrode 14 andthe third electrode 15. Electric power generated in the secondpiezoelectric body 12 is taken out through the lead wire 16 b (see FIG.2) connected to the second electrode 14 and a lead wire 16 c (see FIG.2) connected to the third electrode 15.

The first to third electrodes 13 to 15 are each formed of a suitableconductive material. The first to third electrodes 13 to 15 can be eachformed of, for example, one of metals, such as Al, Ag, Cu, Pt, Au, Crand Ni, or an alloy containing one or more of those metals.

The first and second piezoelectric bodies 11 and 12 are each formed of asuitable piezoelectric material. The first and second piezoelectricbodies 11 and 12 can be each formed of, for example, a piezoelectricceramic such as a lead zirconate titanate-based ceramic, a piezoelectricresin such as PVDF (PolyVinylidene DiFluoride), or a composite materialmade up of at least one of the piezoelectric ceramic and thepiezoelectric resin, and a resin having no piezoelectricity (i.e.,non-piezoelectric resin). Examples of the non-piezoelectric resin usablein the above-mentioned composite material include a silicone resin, aPPS (Polyphenylene Sulfide) resin, a PBT (Polybutylene Terephtalate)resin, a PTFE (Polytetrafluoroethylene) resin, a PET (PolyethyleneTerephthalate) resin, and a PE (Polyethylene) resin. When thepiezoelectric resin or the composite material is used, the piezoelectricbodies 11 and 12 each having substantially the spiral shape can be moreeasily formed than the case of using only the piezoelectric ceramic forthe reason that a high-temperature firing step can be dispensed with inthe former case. Further, when the composite material is used, an outputof piezoelectricity produced in a local area of the piezoelectric powergeneration device, i.e., of the piezoelectric body, can be adjusted bycontrolling a percentage of the piezoelectric ceramic or thepiezoelectric resin that is contained in the composite material.

FIG. 4 is a schematic plan view to explain the direction of polarizationin each of the first and second piezoelectric bodies 11 and 12. Be itnoted that, in FIG. 4, the direction of polarization in each of thefirst and second piezoelectric bodies 11 and 12 is indicated by anarrow.

As illustrated in FIG. 4, the direction 11 c of polarization in thefirst piezoelectric body 11 and the direction 12 c of polarization inthe second piezoelectric body 12 are both oriented in the radialdirection of the piezoelectric element 10 and are opposed to each other.

More specifically, the direction 11 c of polarization in the firstpiezoelectric body 11 is oriented in the radial direction of thepiezoelectric element 10 from one side toward the other side. Thus, thefirst piezoelectric body 11 is polarized in the radial direction of thepiezoelectric element 10 from the one side toward the other side. As onepractical example, in the first embodiment, the direction 11 c ofpolarization in the first piezoelectric body 11 is oriented inwards inthe radial direction of the piezoelectric element 10.

On the other hand, the direction 12 c of polarization in the secondpiezoelectric body 12 is oriented in the radial direction of thepiezoelectric element 10 from the other side toward the one side. Thus,the second piezoelectric body 12 is polarized in the radial direction ofthe piezoelectric element 10 from the other side toward the one side. Asone practical example, in the first embodiment, the direction 12 c ofpolarization in the second piezoelectric body 12 is oriented outwards inthe radial direction of the piezoelectric element 10.

As illustrated in FIG. 1, the piezoelectric element 10 is disposed on asurface 20 a of a plate-like first fixing member 20. A center-side end10 a of the piezoelectric element 10 is fixed to the first fixing member20. Meanwhile, an outer-side end 10 b of the piezoelectric element 10 isnot fixed to the first fixing member 20, but it is fixed to a secondfixing member 30.

A method of fixing the piezoelectric element 10 to the first and secondfixing members 20 and 30 is not limited to a particular one. Thepiezoelectric element 10 may be fixed to the first and second fixingmembers 20 and 30, for example, by welding or by using an adhesive,bolts, etc.

The second fixing member 30 is displaceable relative to the first fixingmember 20 in a direction x tangential to the outer-side end 10 b of thepiezoelectric element 10. On the other hand, the second fixing member 30is restricted from displacing relative to the first fixing member 20 ina direction y perpendicular to the tangential direction x.

More specifically, the first fixing member 20 includes a plate-likemember 21 and a projected portion 22. The center-side end 10 a of thepiezoelectric element 10 is fixed to one surface of the plate-likemember 21. The projected portion 22 is formed on the one surface of theplate-like member 21. The projected portion 22 is projected from the onesurface of the plate-like member 21 in a direction normal to that onesurface at a position outwards of the outer-side end 10 b of thepiezoelectric element 10 as viewed in the radial direction.

While the projected portion 22 is substantially columnar (cylindrical)in the first embodiment, the shape of the projected portion 22 is notlimited to a column. The projected portion 22 may have substantially theshape of a triangular prism, a polygonal column, a cone, or a truncatedcone, for example. Further, the projected portion 22 and the plate-likemember 21 may be integral with each other or separate from each other.

The second fixing member 30 is disposed on the first fixing member 20.The second fixing member 30 is displaceable relative to the first fixingmember 20 in the planar direction of the plate-like member 21 of thefirst fixing member 20. The second fixing member 30 may be displaceableor not displaceable relative to the first fixing member 20 in adirection normal to the plate-like member 21.

The second fixing member 30 includes a fixing member body 31 and firstand second stopper portions 32 and 33 which are connected to the fixingmember body 31. The outer-side end 10 b of the piezoelectric element 10is connected to the fixing member body 31. The fixing member body 31 ispositioned between the outer-side end 10 b of the piezoelectric element10 and the projected portion 22. In the first embodiment, the fixingmember body 31 is formed substantially in a parallelepiped shape, and anend surface of the fixing member body 31 on the side facing theprojected portion 22 defines a contact surface 31 a that is held incontact with the projected portion 22. The contact surface 31 a extendsin the tangential direction x.

The first stopper portion 32 is positioned on one side, denoted by x1,with respect to the projected portion 22 as viewed in the tangentialdirection x. The first stopper portion 32 extends in the direction yfrom the contact surface 31 a toward one side, denoted by y1, in thedirection y. On the other hand, the second stopper portion 33 ispositioned on the other side, denoted by x2, with respect to theprojected portion 22 as viewed in the tangential direction x. The secondstopper portion 33 also extends in the direction y from the contactsurface 31 a toward the side y1 in the direction y.

In the first embodiment, therefore, the second fixing member 30 and theouter-side end 10 b of the piezoelectric element 10, which is fixed tothe second fixing member 30, are replaceable in the tangential directionx through a distance between the first stopper portion 32 and the secondstopper portion 33, as viewed in the tangential direction x, relative tothe first fixing member 20 and the center-side end 10 a of thepiezoelectric element 10, which is fixed to the first fixing member 20,but they are not displaceable in the direction y.

While the first embodiment is described, by way of example, inconnection with the case where the end surface of the fixing member body31 on the side facing the projected portion 22 is held in contact withthe projected portion 22, a clearance may be left between the endsurface of the fixing member body 31 on the side facing the projectedportion 22 and the projected portion 22. When a clearance is leftbetween the end surface of the fixing member body 31 on the side facingthe projected portion 22 and the projected portion 22, the second fixingmember 30 is apt to displace in the perpendicular direction y relativeto the first fixing member 20. In such a case, therefore, the size ofthe clearance between the end surface of the fixing member body 31 onthe side facing the projected portion 22 and the projected portion 22 ispreferably set to be about 0.2 mm or less.

In the piezoelectric power generation device 1 according to the firstembodiment, when vibration is applied to the piezoelectric powergeneration device 1, stresses are imposed to the first and secondpiezoelectric bodies 11 and 12, thus causing the first and secondpiezoelectric bodies 11 and 12 to deform. As a result, electric power isgenerated in the first and second piezoelectric bodies 11 and 12 and istaken out through the first to third electrodes 13 to 15.

Linear reciprocating vibrations applied to the piezoelectric powergeneration device 1 include vibration reciprocating in the tangentialdirection x illustrated in FIG. 4, vibration reciprocating in theperpendicular direction y that is perpendicular to the tangentialdirection x, and vibration reciprocating in a direction inclined to boththe tangential direction x and the perpendicular direction y. Thevibration reciprocating in the direction inclined to both the tangentialdirection x and the perpendicular direction y can be regarded as aresultant vibration of the vibration in the tangential direction x andthe vibration in the perpendicular direction y. Therefore, the linearreciprocating vibrations applied to the piezoelectric power generationdevice 1 can be essentially divided into two main vibrations, i.e., thevibration in the tangential direction x and the vibration in theperpendicular direction y.

For example, when the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10, differentstresses are imposed to the first and second piezoelectric bodies 11 and12 between the right side and the left side of a straight line L1passing a center C of the piezoelectric element 10 and the outer-sideend 10 b of the piezoelectric element 10. FIGS. 5A and 5B illustrate thestresses imposed to a portion A which is located on the right side ofthe straight line L1 in FIG. 4 and the stresses imposed to a portion Bwhich is located on the left side of the straight line L1 in FIG. 4,respectively, when the outer-side end 10 b of the piezoelectric element10 is displaced toward the side y1 in the perpendicular direction y.Also, FIGS. 6A and 6B illustrate the stresses imposed to the portion Awhich is located on the right side of the straight line L1 in FIG. 4 andthe stresses imposed to the portion B which is located on the left sideof the straight line L1 in FIG. 4, respectively, when the outer-side end10 b of the piezoelectric element 10 is displaced toward the side x2 inthe tangential direction x.

First, the case where the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10 will be describedbelow by primarily referring to FIG. 4 and FIGS. 5A and 5B. FIG. 5A is aschematic explanatory view illustrating the direction of polarization inthe portion A which is located on the right side of the straight line L1in FIG. 4 and the stresses imposed to the right-side portion A. FIG. 5Bis a schematic explanatory view illustrating the direction ofpolarization in the portion B which is located on the left side of thestraight line L1 in FIG. 4 and the stresses imposed to the left-sideportion B.

When the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side y1 in the perpendicular direction y, theright-side portion A is deformed in a direction in which the curvatureincreases. As illustrated in FIG. 5A, therefore, tensile stress isimposed to the first piezoelectric body 11 and compressive stress isimposed to the second piezoelectric body 12.

Also, when the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side y1 in the perpendicular direction y, theleft-side portion B is deformed in a direction in which the curvaturedecreases. As illustrated in FIG. 5B, therefore, compressive stress isimposed to the first piezoelectric body 11 and tensile stress is imposedto the second piezoelectric body 12.

Thus, when the reciprocating vibration in the perpendicular direction yis applied to the piezoelectric element 10, the directions of thestresses imposed to the first piezoelectric body 11 are opposed betweenthe right side and the left side of the straight line L1. In the firstembodiment, the first piezoelectric body 11 is polarized in the radialdirection of the piezoelectric element 10 from the outer side toward theinner side thereof in both the right side and the left side of thestraight line L1. Accordingly, voltages having different polarities aregenerated in the right-side portion A and the left-side portion B of thefirst piezoelectric body 11. As illustrated in FIGS. 5A and 5B, forexample, when a plus voltage is generated in the right-side portion A ofthe first piezoelectric body 11, a minus voltage is generated in theleft-side portion B of the first piezoelectric body 11. As a result, thevoltage generated in the right-side portion A of the first piezoelectricbody 11 and the voltage generated in the left-side portion B of thefirst piezoelectric body 11 cancel each other out. Hence, powergeneration efficiency in the first piezoelectric body 11 reduces.

Similarly, when the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10, the directionsof the stresses imposed to the second piezoelectric body 12 are opposedbetween the right side and the left side of the straight line L1. In thefirst embodiment, the second piezoelectric body 12 is polarized in theradial direction of the piezoelectric element 10 from the inner sidetoward the outer side thereof in both the right side and the left sideof the straight line L1. Accordingly, voltages having differentpolarities are generated in the right-side portion A and the left-sideportion B of the second piezoelectric body 12. As illustrated in FIGS.5A and 5B, for example, when a plus voltage is generated in theright-side portion A of the second piezoelectric body 12, a minusvoltage is generated in the left-side portion B of the secondpiezoelectric body 12. As a result, the voltage generated in theright-side portion A of the second piezoelectric body 12 and the voltagegenerated in the left-side portion B of the second piezoelectric body 12cancel each other out. Hence, power generation efficiency in the secondpiezoelectric body 12 reduces.

Next, the case where the reciprocating vibration in the tangentialdirection x is applied to the piezoelectric element 10 will be describedbelow by primarily referring to FIG. 4 and FIGS. 6A and 6B. FIG. 6A is aschematic explanatory view illustrating the direction of polarization inthe portion A which is located on the right side of the straight line L1in FIG. 4 and the stresses imposed to the right-side portion A. FIG. 6Bis a schematic explanatory view illustrating the direction ofpolarization in the portion B which is located on the left side of thestraight line L1 in FIG. 4 and the stresses imposed to the left-sideportion B.

When the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side x2 in the tangential direction x, theright-side portion A and the left-side portion B are both deformed in adirection in which the curvature decreases. As illustrated in FIGS. 6Aand 6B, therefore, compressive stress is imposed to the firstpiezoelectric body 11 and tensile stress is imposed to the secondpiezoelectric body 12 in both the right-side portion A and the left-sideportion B.

Thus, when the reciprocating vibration in the tangential direction x isapplied to the piezoelectric element 10, the directions of the stressesimposed to the first piezoelectric body 11 are the same on both theright side and the left side of the straight line L1. In the firstembodiment, the first piezoelectric body 11 is polarized in the radialdirection of the piezoelectric element 10 from the outer side toward theinner side in both the right side and the left side of the straight lineL1. Accordingly, voltages having the same polarity are generated in theright-side portion A and the left-side portion B of the firstpiezoelectric body 11. As illustrated in FIGS. 6A and 6B, for example,when a minus voltage is generated in the right-side portion A of thefirst piezoelectric body 11, a minus voltage is also generated in theleft-side portion B of the first piezoelectric body 11.

Further, the second piezoelectric body 12 is polarized in the radialdirection of the piezoelectric element 10 from the outer side to theinner side thereof in both the right side and the left side of thestraight line L1. Accordingly, voltages having the same polarity aregenerated in the right-side portion A and the left-side portion B of thesecond piezoelectric body 12. As illustrated in FIGS. 6A and 6B, forexample, when a minus voltage is generated in the right-side portion Aof the second piezoelectric body 12, a minus voltage is also generatedin the left-side portion B of the second piezoelectric body 12.

As a result, unlike the case where the reciprocating vibration in theperpendicular direction y is applied to the outer-side end 10 b of thepiezoelectric element 10, the voltage generated in the right-sideportion A and the voltage generated in the left-side portion B do notcancel each other out. Hence, a high level of power generationefficiency can be obtained.

As seen from the above description, when the first piezoelectric body 11is polarized in the radial direction from one side toward the other sideand the second piezoelectric body 12 is polarized in the radialdirection from the other side toward the one side as in the firstembodiment, a high level of power generation efficiency can be obtainedby applying the reciprocating vibration in the direction x tangential tothe outer-side end 10 b of the piezoelectric element 10.

According to the first embodiment, as described above, the second fixingmember 30 illustrated in FIG. 1 is displaceable relative to the firstfixing member 20 in the tangential direction x, but the displacement ofthe second fixing member 30 relative to the first fixing member 20 inthe perpendicular direction y is restricted with the contact surface 31a contacting the projected portion 22. Therefore, the second fixingmember 30 is allowed to reciprocatingly vibrate relative to the firstfixing member 20 just in a direction that is parallel to the tangentialdirection x. Hence, the piezoelectric power generation device 1according to the first embodiment can realize a high level of powergeneration efficiency.

Further, the first and second stopper portions 32 and 33 are provided inthe first embodiment. With the provision of the first and second stopperportions 32 and 33, the second fixing member 30 is restricted fromexcessively displacing relative to the first fixing member 20 in thetangential direction x. As a result, the piezoelectric element 10 iseffectively protected against damage.

Still further, according to the first embodiment, since thepiezoelectric element 10 is substantially in the spiral shape, thedistance between both the fixed ends of the piezoelectric element 10 canbe increased without enlarging the size of the piezoelectric powergeneration device 1. Stated another way, the piezoelectric element 10having a longer size can be compactly stored in a smaller space byforming the piezoelectric element 10 substantially in the spiral shape.As a result, production of electricity by the piezoelectric powergeneration device 1 can be increased without enlarging the size of thepiezoelectric power generation device 1. Moreover, the resonancefrequency of the piezoelectric element 10 can be lowered withoutenlarging the size of the piezoelectric power generation device 1. It ishence possible to realize the piezoelectric power generation device 1which has a small size and which can generate electric power even withvibrations caused by a motion of a human body, etc. at frequencies aslow as about 20 Hz or less, for example.

In addition, since the piezoelectric element 10 is substantially in thespiral shape, stresses are less apt to concentrate in a particularportion of the piezoelectric element 10. Consequently, the piezoelectricelement 10 is less susceptible to damage with vibrations of thepiezoelectric element 10.

Furthermore, according to the first embodiment, since the first andsecond piezoelectric bodies 11 and 12 are each polarized in the radialdirection of the piezoelectric element 10, the first to third electrodes13 to 15 and the lead wires 16 a to 16 c can be formed in simpleconstructions.

Second Embodiment

The first embodiment has been described above in connection with thecase where the projected portion 22 is formed on the first fixing member20 and the contact surface 31 a is formed in the second fixing member 30such that the second fixing member 30 is displaceable in the tangentialdirection x relative to the first fixing member 20, but the displacementof the second fixing member 30 in the perpendicular direction y relativeto the first fixing member 20 is restricted with the contact surface 31a contacting the projected portion 22. However, the present invention isnot limited to the above-described arrangement. For example, a pluralityof piezoelectric elements 10 may be arranged such that the second fixingmember 30 is displaceable in the tangential direction x relative to thefirst fixing member 20, but the second fixing member 30 is restrictedfrom displacing in the perpendicular direction y relative to the firstfixing member 20. Examples of such an arrangement will be describedbelow in second and third embodiments. Be it noted that, in thefollowing descriptions of the second and third embodiments, componentshaving functions substantially common to those of the components in thefirst embodiment are denoted by common reference characters anddescriptions of those components are omitted. Also, FIGS. 2 to 6 arecontinuously referred to in the following descriptions of the second andthird embodiments in common.

FIG. 7 is a schematic perspective view of a piezoelectric powergeneration device 1 a according to the second embodiment. FIG. 8 is aschematic plan view of the piezoelectric power generation device 1 aaccording to the second embodiment. As illustrated in FIGS. 7 and 8, thepiezoelectric power generation device 1 a according to the secondembodiment includes two piezoelectric elements 10. In the secondembodiment, of the two piezoelectric elements 10, one on the side y2 iscalled a piezoelectric element 10A and the other on the side y1 iscalled a piezoelectric element 10B.

In the second embodiment, the piezoelectric element 10A and thepiezoelectric element 10B have substantially the same structure.Therefore, the resonance frequency of the piezoelectric element 10A andthe resonance frequency of the piezoelectric element 10B are equal toeach other.

The piezoelectric element 10A and the piezoelectric element 10B arearranged so as to line up in the perpendicular direction y. Thepiezoelectric element 10A and the piezoelectric element 10B are arrangedon the surface 20 a of the first fixing member 20 in such a state thatwinding directions of their spirals are opposed to each other. Herein,the term “winding direction of the piezoelectric element” means thedirection in which the piezoelectric element extends from thecenter-side end toward the outer-side end-thereof. Thus, the windingdirection of each piezoelectric element having substantially the spiralshape is right-handed or left-handed. In the second embodiment, asviewed from above the surface 20 a of the first fixing member 20, thewinding direction of the piezoelectric element 10A is left-handed (i.e.,counterclockwise) and the winding direction of the piezoelectric element10B is right-handed (i.e., clockwise).

The second fixing member 30 is arranged on the surface 20 a of the firstfixing member 20 between the piezoelectric element 10A and thepiezoelectric element 10B. The second fixing member 30 is displaceablerelative to the first fixing member 20 at least in the planar directionof the first fixing member 20. In the second embodiment, the secondfixing member 30 is displaceable relative to the first fixing member 20in the planar direction of the first fixing member 20 and a directionnormal to the surface 20 a of the first fixing member 20. Thus, in thesecond embodiment, the second fixing member 30 is not fixed to the firstfixing member 20.

The second fixing member 30 is fixed to both the piezoelectric element10A and the piezoelectric element 10B. Specifically, a portion of thepiezoelectric element 10A, which is positioned closest to thepiezoelectric element 10B, and a portion of the piezoelectric element10B, which is positioned closest to the piezoelectric element 10A, arefixed to the second fixing member 30 on a straight line L2 (see FIG. 8)that passes a center of the piezoelectric element 10A and a center ofthe piezoelectric element 10B. In more detail, in the second embodiment,an outer-side end 10 b of the piezoelectric element 10A is positionedclosest to the piezoelectric element 10B on the straight line L2, and anouter-side end 10 b of the piezoelectric element 10B is positionedclosest to the piezoelectric element 10A on the straight line L2.Therefore, the outer-side end 10 b of the piezoelectric element 10A andthe outer-side end 10 b of the piezoelectric element 10B are fixed tothe second fixing member 30.

With such an arrangement, the second fixing member 30 is restricted fromdisplacing relative to the first fixing member 20 in the perpendiculardirection y. On the other hand, the second fixing member 30 isdisplaceable relative to the first fixing member 20 in the tangentialdirection x. Thus, in the second embodiment, the piezoelectric element10B provided in addition to the piezoelectric element 10A functions as arestriction member for restricting the displacement of the second fixingmember 30 relative to the first fixing member 20 in the perpendiculardirection y, like the projected portion 22 in the first embodiment.Accordingly, the outer-side end 10 b of the piezoelectric element 10Aand the outer-side end 10 b of the piezoelectric element 10B are bothdisplaceable just in the tangential direction x relative to the firstfixing member 20 to which a center-side end 10 a of the piezoelectricelement 10A and a center-side end 10 a of the piezoelectric element 10Bare fixed. As a result, a high level of power generation efficiency canbe realized as in the first embodiment.

According to the second embodiment, in particular, since twopiezoelectric elements 10A and 10B are provided within one piezoelectricpower generation device 1 a, a higher level of power generationefficiency can be realized without enlarging the size of thepiezoelectric power generation device 1 a.

Further, fixing two piezoelectric elements 10 to the second fixingmember 30 as in the second embodiment means that the second fixingmember 30 is fixed at two ends. Therefore, the displacement of thesecond fixing member 30 relative to the first fixing member 20 in thetangential direction x can be stabilized.

Particularly, according to the second embodiment, since the resonancefrequency of the piezoelectric element 10A and the resonance frequencyof the piezoelectric element 10B are equal to each other, thedisplacement of the second fixing member 30 relative to the first fixingmember 20 in the tangential direction x can be more stabilized. Hence,the displacement of the second fixing member 30 relative to the firstfixing member 20 can be increased. As a result, a higher level of powergeneration efficiency can be realized.

Further, according to the second embodiment, since the outer-side end 10b of the piezoelectric element 10A and the outer-side end 10 b of thepiezoelectric element 10B are fixed to the second fixing member 30, theentirety of the piezoelectric elements 10A and 10B is deformed to alarger extent upon the second fixing member 30 displacing relative tothe first fixing member 20. As a result, a higher level of powergeneration efficiency can be realized.

Additionally, when a plurality of piezoelectric elements 10 are providedas in the second embodiment, the stress imposed to one piezoelectricelement can be reduced. Hence, the piezoelectric element 10 can beeffectively protected against damage.

Third Embodiment

The second embodiment has been described in connection with the casewhere the piezoelectric element 10 is provided two. In more detail, thesecond embodiment has been described in connection with the case ofproviding one piezoelectric element pair which includes twopiezoelectric elements 10 having the winding directions opposed to eachother. However, the present invention is not limited to such anarrangement.

As illustrated in FIG. 9, by way of example, a piezoelectric elementpair 10C made up of the piezoelectric element 10A and the piezoelectricelement 10B, which have the winding directions opposed to each other,may be provided so as to lie side by side in the tangential direction x.More specifically, in a third embodiment illustrated in FIG. 9, thepiezoelectric element pair 10C is provided so as to lie side by side inthe tangential direction x. The third embodiment can also realize ahigher level of power generation efficiency as in the second embodimentdescribed above.

According to the third embodiment, in particular, since fourpiezoelectric elements 10 are provided within one piezoelectric powergeneration device, an even higher level of power generation efficiencycan be realized without enlarging the size of the piezoelectric powergeneration device.

Fourth Embodiment

FIG. 10 is a schematic perspective view of a piezoelectric powergeneration device 1 b according to a fourth embodiment. As illustratedin FIG. 10, the piezoelectric power generation device 1 b includes apiezoelectric element 10 and first and second fixing members 20 and 30.The piezoelectric power generation device 1 b generates electric powerby utilizing vibration of the piezoelectric element 10 in the d31 mode.

The piezoelectric element 10 is formed substantially in a spiral(coiled) shape. More specifically, the piezoelectric element 10helically extends from a center of the spiral in the tangentialdirection perpendicular to the radial direction while graduallyincreasing the diameter of the spiral. A width W (see FIG. 12) of thepiezoelectric element 10 is smaller than a height H (see FIG. 12) of thepiezoelectric element 10.

As illustrated in FIGS. 11 and 12, the piezoelectric element 10 includesfirst and second piezoelectric bodies 11 and 12, and first to thirdelectrodes 13, 14 and 15. The first piezoelectric body 11 is formedsubstantially in the spiral shape. As illustrated in FIG. 12, a firstelectrode 13 is formed on an inner surface 11 a of the firstpiezoelectric body 11 as viewed in the radial direction. On the otherhand, a second electrode 14 is formed on an outer surface 11 b of thefirst piezoelectric body 11 as viewed in the radial direction. A secondpiezoelectric body 12 is disposed on an outer surface 14 a of the secondelectrode 14 as viewed in the radial direction. A third electrode 15 isformed on an outer surface 12 a of the second piezoelectric body 12 asviewed in the radial direction. Stated another way, the first electrode13, the first piezoelectric body 11, the second electrode 14, the secondpiezoelectric body 12, and the third electrode 15 are successivelyformed in the named order in the radial direction of the piezoelectricelement 10 from the inner side toward the outer side thereof. Thus, thefirst piezoelectric body 11 is sandwiched between the first electrode 13and the second electrode 14. Electric power generated in the firstpiezoelectric body 11 is taken out through a lead wire 16 a (see FIG.11) connected to the first electrode 13 and a lead wire 16 b (see FIG.11) connected to the second electrode 14. On the other hand, the secondpiezoelectric body 12 is sandwiched between the second electrode 14 andthe third electrode 15. Electric power generated in the secondpiezoelectric body 12 is taken out through the lead wire 16 b (see FIG.11) connected to the second electrode 14 and a lead wire 16 c (see FIG.11) connected to the third electrode 15.

The first to third electrodes 13 to 15 are each formed of a suitableconductive material. The first to third electrodes 13 to 15 can be eachformed of, for example, one of metals, such as Al, Ag, Cu, Pt, Au, Crand Ni, or an alloy containing one or more of those metals.

The first and second piezoelectric bodies 11 and 12 are each formed of asuitable piezoelectric material. The first and second piezoelectricbodies 11 and 12 can be each formed of, for example, a piezoelectricceramic such as a lead zirconate titanate-based ceramic.

FIG. 13 is a schematic plan view to explain the direction ofpolarization in each of the first and second piezoelectric bodies 11 and12. Be it noted that, in FIG. 13, the direction of polarization in eachof the first and second piezoelectric bodies 11 and 12 is indicated byan arrow.

As illustrated in FIG. 13, the direction 11 c of polarization in thefirst piezoelectric body 11 and the direction 12 c of polarization inthe second piezoelectric body 12 are both oriented in the radialdirection of the piezoelectric element 10. The direction 11 c ofpolarization in the first piezoelectric body 11 and the direction 12 cof polarization in the second piezoelectric body 12 are opposed to eachother. Further, in the fourth embodiment, the direction of polarizationis reversed between one portion of the first piezoelectric body 11,which is located on one side of a straight line L1 passing the center ofthe piezoelectric element 10 and the outer-side end of the piezoelectricelement 10, and the other portion of the first piezoelectric body 11,which is located on the other side of the straight line L1. Also, thedirection of polarization is reversed between one portion of the secondpiezoelectric body 12, which is located on the one side of the straightline L1, and the other portion of the second piezoelectric body 12,which is located on the other side of the straight line L1.

More specifically, as illustrated in FIG. 13, the direction 11 c ofpolarization in a portion of the first piezoelectric body 11, which islocated on the right side of the straight line L1, is oriented in theradial direction of the piezoelectric element 10 from one side towardthe other side. On the other hand, as illustrated in FIG. 13, thedirection 11 c of polarization in a portion of the first piezoelectricbody 11, which is located on the left side of the straight line L1, isoriented in the radial direction of the piezoelectric element 10 fromthe other side toward the one side. As one practical example, in thefourth embodiment, the direction 11 c of polarization in the portion ofthe first piezoelectric body 11, which is located on the right side ofthe straight line L1, is oriented inwards in the radial direction andthe direction 11 c of polarization in the portion of the firstpiezoelectric body 11, which is located on the left side of the straightline L1, is oriented outwards in the radial direction.

Further, as illustrated in FIG. 13, the direction 12 c of polarizationin a portion of the second piezoelectric body 12, which is located onthe right side of the straight line L1, is oriented in the radialdirection of the piezoelectric element 10 from the other side toward theone side. On the other hand, as illustrated in FIG. 13, the direction 12c of polarization in a portion of the second piezoelectric body 12,which is located on the left side of the straight line L1, is orientedin the radial direction of the piezoelectric element 10 from the oneside toward the other side. As one practical example, in the fourthembodiment, the direction 12 c of polarization in the portion of thesecond piezoelectric body 12, which is located on the right side of thestraight line L1, is oriented outwards in the radial direction and thedirection 12 c of polarization in the portion of the secondpiezoelectric body 12, which is located on the left side of the straightline L1, is oriented inwards in the radial direction.

A method of polarizing the first and second piezoelectric bodies 11 and12 as in the fourth embodiment is not limited to a particular one. Forexample, the first and second piezoelectric bodies 11 and 12 can bepolarized as follows. First, the piezoelectric element 10 is formed insuch a state that the first to third electrodes 13 to 15 are each cutbetween the right side and the left side of the straight line L1illustrated in FIG. 13. In that state, the first and secondpiezoelectric bodies 11 and 12 are each polarized as desired.Thereafter, cut right and left portions of each of the first to thirdelectrodes 13 to 15 are interconnected.

As illustrated in FIG. 10, the piezoelectric element 10 is disposed on asurface 20 a of a plate-like first fixing member 20. A center-side end10 a of the piezoelectric element 10 is fixed to the first fixing member20. Meanwhile, an outer-side end 10 b of the piezoelectric element 10 isnot fixed to the first fixing member 20, but it is fixed to a secondfixing member 30.

A method of fixing the piezoelectric element 10 to the first and secondfixing members 20 and 30 is not limited to a particular one. Thepiezoelectric element 10 may be fixed to the first and second fixingmembers 20 and 30, for example, by welding or by using an adhesive,bolts, etc.

The second fixing member 30 is displaceable relative to the first fixingmember 20 in a direction y perpendicular to the outer-side end 10 b ofthe piezoelectric element 10, i.e., in a direction in which the straightline L1 extends. On the other hand, the second fixing member 30 isrestricted from displacing relative to the first fixing member 20 in atangential direction x.

More specifically, the first fixing member 20 includes a plate-likemember 21 and a projected portion 22. The center-side end 10 a of thepiezoelectric element 10 is fixed to one surface of the plate-likemember 21. The projected portion 22 is formed on the one surface of theplate-like member 21. The projected portion 22 is projected from the onesurface of the plate-like member 21 in a direction normal to that onesurface at a position outwards of the outer-side end 10 b of thepiezoelectric element 10 as viewed in the radial direction.

While the projected portion 22 is substantially columnar (cylindrical)in the fourth embodiment, the shape of the projected portion 22 is notlimited to a column. The projected portion 22 may have substantially theshape of a triangular prism, a polygonal column, a cone, or a truncatedcone, for example. Further, the projected portion 22 and the plate-likemember 21 may be integral with each other or separate from each other.

The second fixing member 30 is disposed on the first fixing member 20.The second fixing member 30 includes a fixing member body 31 and firstand second guide portions 35 and 36. The outer-side end 10 b of thepiezoelectric element 10 is connected to the fixing member body 31. Thefixing member body 31 is positioned between the outer-side end 10 b ofthe piezoelectric element 10 and the projected portion 22.

Each of the first and second guide portions 35 and 36 extends parallelto the perpendicular direction y from the fixing member body 31 towardthe side y1 in the perpendicular direction y. The first guide portion 35is positioned on the side x1 in the tangential direction x with respectto the projected portion 22. On the other hand, the second guide portion36 is positioned on the side x2 in the tangential direction x withrespect to the projected portion 22. In the fourth embodiment, the firstand second guide portions 35 and 36 are held in contact at their contactsurfaces 35 a and 36 a with the projected portion 22.

In the fourth embodiment, therefore, the second fixing member 30 and theouter-side end 10 b of the piezoelectric element 10, which is fixed tothe second fixing member 30, are displaceable in the perpendiculardirection y relative to the first fixing member 20 and the center-sideend 10 a of the piezoelectric element 10, which is fixed to the firstfixing member 20, but they are not displaceable in the tangentialdirection x.

While the fourth embodiment is described, by way of example, inconnection with the case where the first and second guide portions 35and 36 are each held in contact with the projected portion 22, aclearance may be left between each of the first and second guideportions 35 and 36 and the projected portion 22. When a clearance isleft between each of the first and second guide portions 35 and 36 andthe projected portion 22, the second fixing member 30 is apt to displacein the tangential direction x relative to the first fixing member 20. Insuch a case, therefore, the size of the clearance between each of thefirst and second guide portions 35 and 36 and the projected portion 22is preferably set to be about 0.2 mm or less.

In the piezoelectric power generation device 1 b according to the fourthembodiment, when vibration is applied to the piezoelectric powergeneration device 1 b, stresses are imposed to the first and secondpiezoelectric bodies 11 and 12, thus causing the first and secondpiezoelectric bodies 11 and 12 to deform. As a result, electric power isgenerated in the first and second piezoelectric bodies 11 and 12 and istaken out through the first to third electrodes 13 to 15.

Linear reciprocating vibrations applied to the piezoelectric powergeneration device 1 b include vibration reciprocating in the tangentialdirection x illustrated in FIG. 13, vibration reciprocating in theperpendicular direction y that is perpendicular to the tangentialdirection x, and vibration reciprocating in a direction inclined to boththe tangential direction x and the perpendicular direction y. Thevibration reciprocating in the direction inclined to both the tangentialdirection x and the perpendicular direction y can be regarded as aresultant vibration of the vibration in the tangential direction x andthe vibration in the perpendicular direction y. Therefore, the linearreciprocating vibrations applied to the piezoelectric power generationdevice 1 b can be essentially divided into two main vibrations, i.e.,the vibration in the tangential direction x and the vibration in theperpendicular direction y.

For example, when the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10, differentstresses are imposed to the first and second piezoelectric bodies 11 and12 between the right side and the left side of the straight line L1passing the center C of the piezoelectric element 10 and the outer-sideend 10 b of the piezoelectric element 10. FIGS. 14A and 14B illustratethe stresses imposed to a portion A which is located on the right sideof the straight line L1 in FIG. 13 and the stresses imposed to a portionB which is located on the left side of the straight line L1 in FIG. 13,respectively, when the outer-side end 10 b of the piezoelectric element10 is displaced toward the side y1 in the perpendicular direction y.Also, FIGS. 15A and 15B illustrate the stresses imposed to the portion Awhich is located on the right side of the straight line L1 in FIG. 13and the stresses imposed to the portion B which is located on the leftside of the straight line L1 in FIG. 13, respectively, when theouter-side end 10 b of the piezoelectric element 10 is displaced towardthe side x2 in the tangential direction x.

First, the case where the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10 will be describedbelow by primarily referring to FIG. 13 and FIGS. 14A and 14B. FIG. 14Ais a schematic explanatory view illustrating the direction ofpolarization in the portion A which is located on the right side of thestraight line L1 in FIG. 13 and the stresses imposed to the right-sideportion A. FIG. 14B is a schematic explanatory view illustrating thedirection of polarization in the portion B which is located on the leftside of the straight line L1 in FIG. 13 and the stresses imposed to theleft-side portion B.

When the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side y1 in the perpendicular direction y, theright-side portion A is deformed in a direction in which the curvatureincreases. As illustrated in FIG. 14A, therefore, tensile stress isimposed to the first piezoelectric body 11 and compressive stress isimposed to the second piezoelectric body 12.

Also, when the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side y1 in the perpendicular direction y, theleft-side portion B is deformed in a direction in which the curvaturedecreases. As illustrated in FIG. 14B, therefore, compressive stress isimposed to the first piezoelectric body 11 and tensile stress is imposedto the second piezoelectric body 12.

Thus, when the reciprocating vibration in the perpendicular direction yis applied to the piezoelectric element 10, the directions of thestresses imposed to the first piezoelectric body 11 are opposed betweenthe right side and the left side of the straight line L1. In the fourthembodiment, the direction 11 c of polarization in the firstpiezoelectric body 11 is reversed between the right side and the leftside of the straight line L1. Accordingly, voltages having the samepolarity are generated in the right-side portion A and the left-sideportion B of the first piezoelectric body 11. As illustrated in FIGS.14A and 14B, for example, when a plus voltage is generated in theright-side portion A of the first piezoelectric body 11, a plus voltageis also generated in the left-side portion B of the first piezoelectricbody 11. As a result, the voltage generated in the right-side portion Aand the voltage generated in the left-side portion B do not cancel eachother out, whereby a high level of power generation efficiency can beobtained.

Similarly, when the reciprocating vibration in the perpendiculardirection y is applied to the piezoelectric element 10, the directionsof the stresses imposed to the second piezoelectric body 12 are opposedbetween the right side and the left side of the straight line L1. In thefourth embodiment, the direction 12 c of polarization in the secondpiezoelectric body 12 is reversed between the right side and the leftside of the straight line L1. Accordingly, voltages having the samepolarity are generated in the right-side portion A and the left-sideportion B of the second piezoelectric body 12. As illustrated in FIGS.14A and 14B, for example, when a plus voltage is generated in theright-side portion A of the second piezoelectric body 12, a plus voltageis also generated in the left-side portion B of the second piezoelectricbody 12. As a result, the voltage generated in the right-side portion Aand the voltage generated in the left-side portion B do not cancel eachother out, whereby a high level of power generation efficiency can beobtained.

Next, the case where the reciprocating vibration in the tangentialdirection x is applied to the piezoelectric element 10 will be describedbelow by primarily referring to FIG. 13 and FIGS. 15A and 15B. FIG. 15Ais a schematic explanatory view illustrating the direction ofpolarization in the portion A which is located on the right side of thestraight line L1 in FIG. 13 and the stresses imposed to the right-sideportion A. FIG. 15B is a schematic explanatory view illustrating thedirection of polarization in the portion B which is located on the leftside of the straight line L1 in FIG. 13 and the stresses imposed to theleft-side portion B.

When the outer-side end 10 b of the piezoelectric element 10 isdisplaced toward the side x2 in the tangential direction x, theright-side portion A and the left-side portion B are both deformed in adirection in which the curvature decreases. As illustrated in FIGS. 15Aand 15B, therefore, compressive stress is imposed to the firstpiezoelectric body 11 and tensile stress is imposed to the secondpiezoelectric body 12 in both the right-side portion A and the left-sideportion B.

Thus, when the reciprocating vibration in the tangential direction x isapplied to the piezoelectric element 10, the directions of the stressesimposed to the first piezoelectric body 11 are the same on both theright side and the left side of the straight line L1. In the fourthembodiment, the direction 11 c of polarization in the firstpiezoelectric body 11 is reversed between the right side and the leftside of the straight line L1. Accordingly, voltages having differentpolarities are generated in the right-side portion A and the left-sideportion B of the first piezoelectric body 11. As illustrated in FIGS.15A and 15B, for example, when a minus voltage is generated in theright-side portion A of the first piezoelectric body 11, a plus voltageis generated in the left-side portion B of the first piezoelectric body11.

Further, the direction 12 c of polarization in the second piezoelectricbody 12 is reversed between the right side and the left side of thestraight line L1. Accordingly, voltages having different polarities aregenerated in the right-side portion A and the left-side portion B of thesecond piezoelectric body 12. As illustrated in FIGS. 15A and 15B, forexample, when a minus voltage is generated in the right-side portion Aof the second piezoelectric body 12, a plus voltage is generated in theleft-side portion B of the second piezoelectric body 12.

Thus, the voltage generated in the right-side portion A and the voltagegenerated in the left-side portion B cancel each other out, whereby ahigh level of power generation efficiency cannot be obtained.

As seen from the above description, in the fourth embodiment, a highlevel of power generation efficiency can be obtained by applying thereciprocating vibration in the direction y perpendicular to theouter-side end 10 b of the piezoelectric element 10.

According to the fourth embodiment, as described above, the secondfixing member 30 illustrated in FIG. 10 is displaceable in theperpendicular direction y relative to the first fixing member 20, butthe displacement of the second fixing member 30 in the tangentialdirection x relative to the first fixing member 20 is restricted withthe contact surface 35 a and 36 a contacting the projected portion 22.Therefore, the second fixing member 30 is allowed to reciprocatinglyvibrate relative to the first fixing member 20 just in a direction thatis parallel to the perpendicular direction y. Hence, the piezoelectricpower generation device 1 b according to the fourth embodiment canrealize a high level of power generation efficiency.

Further, according to the fourth embodiment, with the fixing member body31 arranged in opposed relation to the projected portion 22, the secondfixing member 30 is restricted from excessively displacing toward theside y1 in the perpendicular direction y relative to the first fixingmember 20. As a result, the piezoelectric element 10 is not excessivelydeformed even when large vibration is applied to the piezoelectricelement 10. Hence, the piezoelectric element 10 is effectively protectedagainst damage.

Still further, according to the fourth embodiment, since thepiezoelectric element 10 is substantially in the spiral shape, thedistance between both the fixed ends of the piezoelectric element 10 canbe increased without enlarging the size of the piezoelectric powergeneration device 1 b. Stated another way, the piezoelectric element 10having a longer size can be compactly stored in a smaller space byforming the piezoelectric element 10 substantially in the spiral shape.As a result, production of electricity by the piezoelectric powergeneration device 1 b can be increased without enlarging the size of thepiezoelectric power generation device 1 b. Moreover, the resonancefrequency of the piezoelectric element 10 can be lowered withoutenlarging the size of the piezoelectric power generation device 1 b. Itis hence possible to realize the piezoelectric power generation device 1b which has a small size and which can generate electric power even withvibrations caused by a motion of a human body, etc. at frequencies aslow as about 20 Hz or less, for example.

In addition, since the piezoelectric element 10 is substantially in thespiral shape, stresses are less apt to concentrate in a particularportion of the piezoelectric element 10. Consequently, the piezoelectricelement 10 is less susceptible to damage with vibrations of thepiezoelectric element 10.

Fifth Embodiment

The fourth embodiment has been described above in connection with thecase where the projected portion 22 is formed on the first fixing member20 and the first and second guide portions 35 and 36 are formed as partof the second fixing member 30 such that the second fixing member 30 isdisplaceable in the perpendicular direction y relative to the firstfixing member 20, but the second fixing member 30 is restricted fromdisplacing in the tangential direction x relative to the first fixingmember 20. However, the present invention is not limited to theabove-described arrangement. For example, a plurality of piezoelectricelements 10 may be arranged such that the second fixing member 30 isdisplaceable in the perpendicular direction y relative to the firstfixing member 20, but the second fixing member 30 is restricted fromdisplacing in the tangential direction x relative to the first fixingmember 20. One example of such an arrangement will be described below ina fifth embodiment. Be it noted that, in the following description ofthe fifth embodiment, components having functions substantially commonto those of the components in the fourth embodiment are denoted bycommon reference characters and descriptions of those components areomitted. Also, FIGS. 11 to 15 are continuously referred to in thefollowing description of the fifth embodiment in common.

FIG. 16 is a schematic perspective view of a piezoelectric powergeneration device 1 c according to the fifth embodiment. As illustratedin FIG. 16, the piezoelectric power generation device 1 c according tothe fifth embodiment includes two piezoelectric elements 10. In thefifth embodiment, of the two piezoelectric elements 10, one on the sidex2 is called a piezoelectric element 10A and the other on the side x1 iscalled a piezoelectric element 10B.

In the fifth embodiment, the piezoelectric element 10A and thepiezoelectric element 10B have substantially the same structure.Therefore, the resonance frequency of the piezoelectric element 10A andthe resonance frequency of the piezoelectric element 10B are equal toeach other.

The piezoelectric element 10A and the piezoelectric element 10B arearranged so as to line up in the tangential direction x. Thepiezoelectric element 10A and the piezoelectric element 10B are arrangedon the surface 20 a of the first fixing member 20 in such a state thatwinding directions of their spirals are opposed to each other. Herein,the term “winding direction of the piezoelectric element” means thedirection in which the piezoelectric element extends from thecenter-side end toward the outer-side end thereof. Thus, the windingdirection of each piezoelectric element having substantially the spiralshape is right-handed or left-handed. In the fifth embodiment, as viewedfrom above the surface 20 a of the first fixing member 20, the windingdirection of the piezoelectric element 10A is left-handed (i.e.,counterclockwise) and the winding direction of the piezoelectric element10B is right-handed (i.e., clockwise).

In the fifth embodiment, the direction of polarization in a portion ofthe first piezoelectric body 11 of the piezoelectric element 10A, whichis located on the side x1 with respect to the straight line L1, and thedirection of polarization in a portion of the first piezoelectric body11 of the piezoelectric element 10B, which is located on the side x2with respect to the straight line L1, are equal to each other. Thedirection of polarization in a portion of the first piezoelectric body11 of the piezoelectric element 10A, which is located on the side x2with respect to the straight line L1, and the direction of polarizationin a portion of the first piezoelectric body 11 of the piezoelectricelement 10B, which is located on the side x1 with respect to thestraight line L1, are equal to each other. Further, the direction ofpolarization in a portion of the second piezoelectric body 12 of thepiezoelectric element 10A, which is located on the side x1 with respectto the straight line L1, and the direction of polarization in a portionof the second piezoelectric body 12 of the piezoelectric element 10B,which is located on the side x2 with respect to the straight line L1,are equal to each other. The direction of polarization in a portion ofthe second piezoelectric body 12 of the piezoelectric element 10A, whichis located on the side x2 with respect to the straight line L1, and thedirection of polarization in a portion of the second piezoelectric body12 of the piezoelectric element 10B, which is located on the side x1with respect to the straight line L1, are equal to each other.

The second fixing member 30 is arranged on the surface 20 a of the firstfixing member 20. The second fixing member 30 is displaceable relativeto the first fixing member 20 at least in the planar direction of thefirst fixing member 20. In the fifth embodiment, the second fixingmember 30 is displaceable relative to the first fixing member 20 in theplanar direction of the first fixing member 20 and a direction normal tothe surface 20 a of the first fixing member 20. Thus, in the fifthembodiment, the second fixing member 30 is not fixed to the first fixingmember 20.

The second fixing member 30 is fixed to the piezoelectric element 10Aand the piezoelectric element 10B. Specifically, the second fixingmember 30 is fixed to a portion of the piezoelectric element 10A, whichis positioned closest to the side y1 in the perpendicular direction y,and to a portion of the piezoelectric element 10B, which is positionedclosest to the side y1 in the perpendicular direction y.

The second fixing member 30 includes a connecting portion 37 and aweight portion 38. The connecting portion 37 interconnects the portionof the piezoelectric element 10A, which is positioned closest to theside y1 in the perpendicular direction y, and the portion of thepiezoelectric element 10B, which is positioned closest to the side y1 inthe perpendicular direction y. In the fifth embodiment, an outer-sideend 10 b of the piezoelectric element 10A is positioned closest to theside y1 in the perpendicular direction y, and an outer-side end 10 b ofthe piezoelectric element 10B is positioned closest to the side y1 inthe perpendicular direction y. Hence, the connecting portion 37interconnects the outer-side end 10 b of the piezoelectric element 10Aand the outer-side end 10 b of the piezoelectric element 10B.

The weight portion 38 is positioned between a center of thepiezoelectric element 10A and a center of the piezoelectric element 10Bin the tangential direction x. The weight portion 38 extends from theconnecting portion 37 toward the side y2 in the perpendicular directiony.

With such an arrangement, the second fixing member 30 is restricted fromdisplacing relative to the first fixing member 20 in the tangentialdirection x. On the other hand, the second fixing member 30 isdisplaceable relative to the first fixing member 20 in the perpendiculardirection y. Thus, in the fifth embodiment, the piezoelectric element10B provided in addition to the piezoelectric element 10A functions as arestriction member for restricting the displacement of the second fixingmember 30 relative to the first fixing member 20 in the tangentialdirection x, like the projected portion 22 in the fourth embodiment.Accordingly, the outer-side end 10 b of the piezoelectric element 10Aand the outer-side end 10 b of the piezoelectric element 10B are bothdisplaceable just in the perpendicular direction y relative to the firstfixing member 20 to which a center-side end 10 a of the piezoelectricelement 10A and a center-side end 10 a of the piezoelectric element 10Bare fixed. As a result, a high level of power generation efficiency canbe realized as in the fourth embodiment.

According to the fifth embodiment, in particular, since twopiezoelectric elements 10 are provided within one piezoelectric powergeneration device 1 c, a higher level of power generation efficiency canbe realized without enlarging the size of the piezoelectric powergeneration device 1 c.

Further, fixing two piezoelectric elements 10 to the second fixingmember 30 as in the fifth embodiment means that the second fixing member30 is fixed at two ends. Therefore, the displacement of the secondfixing member 30 relative to the first fixing member 20 in theperpendicular direction y can be stabilized.

Particularly, according to the fifth embodiment, since the resonancefrequency of the piezoelectric element 10A and the resonance frequencyof the piezoelectric element 10B are equal to each other, thedisplacement of the second fixing member 30 relative to the first fixingmember 20 in the tangential direction x can be more stabilized. Hence,the displacement of the second fixing member 30 relative to the firstfixing member 20 can be increased. As a result, a higher level of powergeneration efficiency can be realized.

Moreover, when a plurality of piezoelectric elements 10 are provided asin the fifth embodiment, the stress imposed to one piezoelectric elementcan be reduced. Hence, the piezoelectric element 10 can be effectivelyprotected against damage.

Additionally, according to the fifth embodiment, since the connectingportion 37 interconnects the outer-side end 10 b of the piezoelectricelement 10A and the outer-side end 10 b of the piezoelectric element10B, the entirety of the piezoelectric elements 10A and 10B is deformedto a larger extent upon the second fixing member 30 displacing relativeto the first fixing member 20. As a result, a higher level of powergeneration efficiency can be realized.

Sixth Embodiment

The first embodiment has been described in connection with the casewhere the first and second piezoelectric bodies 11 and 12 are bothprovided as illustrated in FIG. 4. However, the present invention is notlimited to such an arrangement. For example, one of the first and secondpiezoelectric bodies 11 and 12 may be formed of a non-piezoelectricelastic body other than a piezoelectric body. As illustrated in FIG. 17,by way of example, a non-piezoelectric elastic member 17 substantiallyin a spiral shape may be provided instead of the first piezoelectricbody 11. Even with the case of using the non-piezoelectric elasticmember 17, stresses imposed to the second piezoelectric body 12 are thesame as those imposed to the second piezoelectric body 12 in the firstembodiment. Hence, a high level of power generation efficiency can beobtained as in the first embodiment.

Similarly, at least one of the first and second piezoelectric bodies 11and 12 in at least one of the first and second piezoelectric elements10A and 10B in the second embodiment may be formed of anon-piezoelectric elastic body other than a piezoelectric body.

Materials usable to form the non-piezoelectric elastic member 17 are notlimited to particular ones so long as the used material is other than apiezoelectric body and has elasticity. The non-piezoelectric elasticmember 17 can be formed of, e.g., a metal, a resin, a ceramic, etc.

In the sixth embodiment, the first piezoelectric body 11 may be providedin multiple layers.

Seventh Embodiment

The fourth embodiment has been described in connection with the casewhere the first and second piezoelectric bodies 11 and 12 are bothprovided. However, the present invention is not limited to such anarrangement. For example, one of the first and second piezoelectricbodies 11 and 12 may be formed of a non-piezoelectric elastic body otherthan a piezoelectric body. As illustrated in FIG. 18, by way of example,a non-piezoelectric elastic member 17 substantially in a spiral shapemay be provided instead of the first piezoelectric body 11. Even withthe case of using the non-piezoelectric elastic member 17, stressesimposed to the second piezoelectric body 12 are the same as thoseimposed to the second piezoelectric body 12 in the fourth embodiment.Hence, a high level of power generation efficiency can be obtained as inthe fourth embodiment.

Similarly, at least one of the first and second piezoelectric bodies 11and 12 in at least one of the first and second piezoelectric elements10A and 10B in the fifth embodiment may be formed of a non-piezoelectricelastic body other than a piezoelectric body.

The first and second piezoelectric bodies 11 and 12 are each provided inthe form of a single layer in the above-described embodiments, but thefirst and second piezoelectric bodies 11 and 12 may be each provided inthe form of multiple layers.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

1. A piezoelectric power generation device comprising: a spiral-shapedpiezoelectric element having a center-side end and an outer-side end; afirst fixing member to which the center-side end of the piezoelectricelement is fixed; and a second fixing member to which the outer-side endof the piezoelectric element is fixed, the piezoelectric elementincluding: a first spiral-shaped piezoelectric body polarized in aradial direction from a first side toward a second side thereof; a firstelectrode disposed on the first side of the first spiral-shapedpiezoelectric body; a second electrode disposed on the second side ofthe first spiral-shaped piezoelectric body; and a spiral-shaped memberdisposed on one of the second electrode and the first electrode, whereinthe first and second fixing members are arranged such that the secondfixing member is displaceable relative to the first fixing member in adirection tangential to the outer-side end of the spiral-shapedpiezoelectric element, and the second fixing member is restricted fromdisplacing relative to the first fixing member in a directionperpendicular to the direction tangential to the outer-side end of thespiral-shaped piezoelectric element.
 2. The piezoelectric powergeneration device according to claim 1, wherein the spiral-shaped memberis a second piezoelectric body which is polarized in the radialdirection, and the piezoelectric element further includes a thirdelectrode disposed on a surface of the second piezoelectric body.
 3. Thepiezoelectric power generation device according to claim 1, wherein thespiral-shaped member is an elastic body.
 4. The piezoelectric powergeneration device according to claim 1, wherein the first fixing memberincludes a plate-like member having one surface on which thepiezoelectric element is positioned, and a projected portion projectingfrom the one surface of the plate-like member at a position outwards ofthe outer-side end of the piezoelectric element in the radial direction,the second fixing member is positioned proximal to the piezoelectricelement with respect to the projected portion and has a contact surfaceextending in the direction tangential to the outer-side end of thepiezoelectric element, and the contact surface is held in contact withthe projected portion.
 5. The piezoelectric power generation deviceaccording to claim 4, wherein the second fixing member includes: afixing member body having the contact surface, a first stopper portionconnected to the fixing member body and positioned on a first side ofthe projected portion in the direction tangential to the outer-side endof the piezoelectric element, and a second stopper portion connected tothe fixing member body and positioned on a second side of the projectedportion in the direction tangential to the outer-side end of thepiezoelectric element.